According to Alex Keybl, Firefox’s release manager, the automatic upgrade of Firefox 3.6 to Firefox 12 will take place in early May, although a date has not yet been set.

The decision to push Firefox 3.6 users to a newer edition has been under discussion for several weeks. In late March, Keybl brought it up on a Mozilla planning discussion thread, saying that the proposal was needed to keep users safe while they browsed.

Mozilla issued its final security update for Firefox 3.6 in January and officially retired the browser from support last Tuesday.

Mozilla has given Firefox 3.6 users plenty of warning, telling them several times over the last months that they need to upgrade because of the impending retirement. The newest such message told Firefox 3.6 users that it was their last warning before Mozilla switched on automatic upgrading.

Besides Firefox 12, which launched Tuesday, users can also turn to Firefox Extended Support Release (ESR), the build that Mozilla crafted to soothe customers, including IT administrators who manage Firefox for their workers, who were unhappy with the every-six-week upgrade cadence of the standard browser.

The current version of Firefox ESR is based on Firefox 10, which shipped last December. ESR receives only security updates during its 54-week lifespan. Except for the patches, Firefox ESR won’t change until this November, and will be supported with updates until early February next year.

If users want to keep Firefox 3.6, they must disable updates before the May auto-upgrade. On the Mac, go to the Firefox 3.6 Preferences and then click the Updates tab. Choos Never Check for Updates.”

“Users will be automatically updated unless they have specifically disabled updates,” said Keybl in an email reply to questions. “However, we strongly advise our users to upgrade from Firefox 3.6, as they will no longer receive critical security updates.”

Firefox 3.6 users will soon see this notice, which tells them that their browser will automatically be upgraded to Firefox 12.

In May of last year, Mozilla automatically upgraded Firefox 3.5 to Firefox 3.6 after the former was retired from support, the first time it had used the tactic to rub out an aged browser. It worked: Firefox 3.5’s share fell dramatically.

Firefox 3.6 accounted for a significant chunk of Mozilla’s usage share last month. By metrics company Net Applications’ estimates, 13 percent of all copies of Firefox, or about one-in-eight, were version 3.6.